1. Technical Field of the Invention
The present invention relates generally to an antenna apparatus for use in wristwatch radio communication devices which have, for example, a PHS (Personal Handyphone System) employing a radio frequency higher than a UHF band, and more particularly to an antenna apparatus suitable for wearing on the user""s wrist which is designed to assure stable and reliable communications during conversation and waiting time and a wristwatch radio communication device using the antenna apparatus.
2. Background Art
Recently, mobile telecommunication systems such as mobile telephones and PHSs have been developed rapidly, and portable radio units are being miniaturized. As such portable radio units, there have been proposed wristwatch radio communication devices. As a typical one of the wristwatch radio communication devices, wristwatch pagers are put into practical use. For instance, Japanese Utility Model First Publication No. 5-21536 teaches a helical antenna built in a wristband. Japanese Patent First Publication No. 5-276056 teaches a loop antenna mounted in a wristband. Japanese Patent First Publication No. 6-188809 teaches a loop antenna installed in a cover of a wristwatch casing. Japanese Patent First Publication No. 8-274536 discloses a slot antenna disposed in a wristwatch casing.
Additionally, as a wristwatch radio antenna used in PHSs whose radio frequency is, for example, 1.9 GHz, Japanese Patent First Publication No. 11-55143 proposes a one-wavelength loop antenna installed in a wristwatch cover.
Usually, as a performance index for antennas of mobile telephones such as PHSs, a horizontal pattern average gain (PAG) when the antenna is worn on a body of a person is used. If a person""s body wearing a mobile telephone antenna is positioned at the center of a spherical coordinate system with a head oriented toward the zenith, PAG may be given by an equation below.   PAG  =            1              2        ⁢        π              ⁢                  ∫        0                  2          ⁢          π                    ⁢                        [                                                    G                θ                            ⁢                              (                φ                )                                      +                                                            G                  φ                                ⁢                                  (                  φ                  )                                            XPR                                ]                ⁢                  ⅆ          φ                    
where Gxcex8(xcfx86) and Gxcfx86(xcfx86) are power patterns of vertically and horizontally polarized components in a horizontal plane (X-Y plane), respectively, and XPR is a cross polarization discrimination of an incoming wave received by the antenna, or a power ratio of the vertically polarized component to the horizontally polarized component. A typical XPR of land mobile communication systems in multi-path environmental conditions is known to be 4 to 9 dB meaning that the power of the vertical polarization of an incoming wave is higher than that of the horizontal polarization by 4 to 9 dB. Thus, the above equation indicates an average of the power patterns in a horizontal plane derived by weighting the vertical polarization by the XPR. In the following discussion, a cross polarization discrimination XPR of 6 dB that is a typical value in urban area.
In order to assure a high communication quality, antennas of radio systems such as PHSs require a PAG greater than about xe2x88x926 dBd (half wavelength dipole ratio). Typical telephone units attach great importance in design to the talk position when a speaker is placed close to the user""s ear. The wristwatch radio communication devices, however, must consider two use conditions: one is a condition in which the device worn on the wrist is placed in front of the user""s face for conversation, and the second is a condition in which the device worn on the wrist is held on the side of the waist during a waiting time. In the two conditions, the PAG must be xe2x88x926 dBd or more. The one-wavelength loop antenna built in the wristwatch cover, as described above, is designed to improve the performance of the antenna both in cases in which the cover is opened during the conversation time and in which the cover is closed during the waiting time.
However, the above described antennas for wristwatch pagers are designed to use a radio frequency of less than 300 MHz and have a lower antenna gain usually less than xe2x88x9215 dBd. It is, thus, difficult to use such antennas in wristwatch radio communication devices employing the PHS whose radio frequency is higher than the UHF band.
The one-wavelength loop antenna, as taught in Japanese Patent First Publication No. 11-55143, for use in PHS wristwatch radio communication devices improves the antenna performance during the conversation and waiting times to a certain extent, but basically uses a balanced feed structure, as shown in FIG. 2, that is undesirably complex. Additionally, this antenna is also not optimized in view of characteristics of radiation incorporating the whole of the user""s body, which will lead to a problem that a PAG more than xe2x88x926 dBd cannot be assured both during the conversation and waiting times.
The above one-wavelength loop antenna is so used that the bi-directional directivity thereof is oriented to right and left of the user""s body during the conversation time and has a problem that the radiation toward the front of the user""s body is decreased. The acquisition of vertical polarization requires installation of a feed on a side of the cover which is perpendicular to the wristwatch casing when opened, thus needing a feed cable for connection to a radio circuit arranged in the wristwatch casing. The impedance of the loop antenna when the cover is opened is high, as much as 100 xcexa9, thus resulting in a difficulty in matching with a 50 xcexa9 impedance of the radio circuit.
It is therefore a principal object of the invention to avoid the disadvantages of the prior art.
It is another object of the invention to provide an antenna apparatus which has a simple feed structure to assure high antenna performance both during conversation and waiting times in a portable radio telephone and a wristwatch radio communication device using the same.
According to one aspect of the invention, there is provided an antenna apparatus for use in a wristwatch radio communication device. The antenna apparatus comprises: (a) a loop conductor having a given length, the loop conductor being connected electrically at one end to a ground member installed in the wristwatch radio communication device; and (b) a feed disposed between the other end of the loop conductor and the ground member.
The loop conductor is rectangular and has a circumferential length equivalent to a wavelength of a frequency used in radio communication of the wristwatch radio communication device. The loop conductor has at least one side disposed closer to the ground member installed in the wristwatch radio communication device.
The loop conductor is arranged along a periphery of an openable cover installed on the wristwatch radio communication device. The loop conductor, the ground member, and the feed are connected electrically through a support mechanism designed to support the cover pivotably on a body of the wristwatch radio communication device.
The support mechanism is provided on a side of the body of the wristwatch radio communication device to which a wristband is attached.
The loop conductor is printed on a board installed within the cover of the wristwatch radio communication device.
The board has a matching circuit disposed thereon.
A ground conductor is further disposed within the wristband which is connected electrically to the ground member in the wristwatch radio communication device.
According to another aspect of the invention, there is provided a wristwatch radio communication device which comprises: (a) a wristwatch body having a radio communication circuit; (b) a ground member installed in the wristwatch body; and (c) an antenna. The antenna includes a loop conductor having a given length, connected electrically at one end to the ground member and a feed disposed between the other end of the loop conductor and the ground member.